Light signals from the environment are perceived by specific regulatory photoreceptors in plants and transduced by unknown mechanisms to control growth and development. The long term goal of this research is to dissect the molecular mechanisms by which such signals are transduced and integrated to modulate plant development. The light-regulated early seedling development of Arabidopsis thaliana will be used as the experimental system. In previous studies we have identified seven cop (constitutive photomorphogenic) loci, which when mutated produce dark-grown seedlings with a morphology resembling that of light-grown wild-type seedlings. We propose here to clone and sequence the cop1 locus, by using a T-DNA insertional mutant allele which we have identified. Using the appropriate molecular probes (DNA and antibodies) for blot analysis and in situ localization, we will begin to determine how the wild-type cop1 gene product couples light signals and seedling development at the molecular level. Once we have established the molecular nature of the cop1 gene, we will start to identify components that directly interact with the cop1 gene product. Furthermore, we will also analyze the effect of other photomorphogenic mutations on regulation of cop1 gene expression to define a regulatory hierarchy of genes that control light-regulated seedling development. Accomplishment of these studies will for the first time not only ascertain the nature of a critical component that links light signals to seedling development, but also provide clues as to how the expression of the cop1 gene responds to light signals and regulates plant development.